Such an electric cable is known from EP 1 632 957 A2. This prior art document discloses a screening sheet for the at least one line which screening sheet includes at least one substrate layer of a plastic material and at least one screening layer of an electrically conductive material, in particular metal, which the substrate layer is lined with. The screening layer is provided with spacing gaps for electrical interruption thereof in a longitudinal strip direction with the spacing gaps extending crosswise of the longitudinal strip direction and recurring at longitudinal intervals. Further on the cable includes an external envelope of an insulating material.
The problems the invention deals with can be explained most obviously in conjunction with high-speed data transmission cables, which, however, does not restrict the use of the invention to this purpose.
Customary data transmission cables use several of the above twisted pairs, for example four, which are preferably screened as the category of transmission bandwidth and transmission quality rises. External screening of the twisted pairs as well as screening of the twisted pairs one in relation to the other in a cable are important in this case.
For corresponding specifications of transmission bandwidth and transmission quality to be obtained, U.S. Pat. No. 6,624,359 B2 teaches to provide the twisted pairs with a screening sheet which is comprised of a laminate of a plastic-material substrate layer lined with a screening layer of metal. This document further shows the most varying configurations of how to fold this laminated sheet so that it forms an external screening envelope placed around several twisted pairs. Fundamentally, the screening sheet is designed as a strip of material having a continuous screening layer, for example of aluminum or copper, in the longitudinal direction of the strip.
The above design of an electrically conductive screening layer that is continuous in the longitudinal direction of the cable gives rise to problems of grounding because, given varying potentials at the ends of a line, high potential compensation currents can flow through the screening. They cause malfunction and possibly even damages of equipment connected to such a data transmission cable.
This problem is solved according to the above-mentioned EP 1 632 957 A2 by the strip-type screening sheet comprising spacing gaps in the screening layer which extend somewhat crosswise of the longitudinal direction of the strip, longitudinally recurring at intervals. They serve for electrical interruption of the screening layer in the longitudinal direction of the strip. Consequently, there is no continuous electrically conductive connection in the longitudinal direction of the screening sheet, which completely precludes any flow of potential compensation currents. But although the gaps being small as compared to the rest of the screening surface of the pieces of foil that lie between the spacing gaps, there is some deterioration in the screening properties of the screening sheet which for high-frequency applications might not be acceptable.